There is a known air-conditioning apparatus for carrying out heating under a low outdoor air temperature environment of about −10 degrees C. that performs injection of a gas refrigerant or a two-phase refrigerant into a compressor. However, even in an injection type air-conditioning apparatus, further drop in the outdoor air temperature will cause the heating capacity ratio (the actual exerted capacity over the inherent capacity) to drop.
Additionally, if the low outdoor air temperature drops even further, the evaporating temperature of the refrigeration cycle becomes low and the discharge temperature of the compressor increases, hindering normal operation due to the need to protect the compressor.
Meanwhile, there is a known air-conditioning apparatus that has increased its heating capacity by using a heat source (external heat source) other than the refrigerant flowing in the refrigerant circuit of the refrigeration cycle. For example, there is an air-conditioning apparatus that enables continuous heating operation by securing a heating capacity of a heat pump air-conditioning apparatus by utilizing hot water of a boiler (Patent Literature 1). Furthermore, there is a known air-conditioning apparatus that carries out heating by simultaneously utilizing an air-cooled heat exchanger and a water-cooled heat exchanger, which uses hot water of a boiler, when the outdoor air temperature is low (Patent Literature 2).